Studies involve collaboration on the characterization of neoplastic cells and the study of tumor histogenesis. The aim of the present investigation is to define, by high resolution and immunoelectron microscopy, changes in natural killer (NK) cells and targeted tumor cells which correlate with NK cell activity. Large granular lymphocytes have been identified with cytotoxic activity, probably via secretion of a cytolytic material (cytolysin) which is stored in cytoplasmic granules. Using recently produced antibodies against cytolytic components, it has been possible to localize this cytolysin component to the cytoplasmic granules using colloidal gold probes on thin sections of glycol methacylate embedded tissue. It should now be possible to examine the movement of cytolytic proteins during NK cell attack of targeted tumorigenic cells.